The present invention relates to an Fe-B-R type permanent magnet with excellent magnetic properties and high corrosion resistance, and more specifically to a Fe-B-R type permanent magnet stable in magnetic properties; in particular, small in deterioration rate from the initial magnetic properties after having been kept in an atmosphere of a temperature of 80.degree. C. and a relative humidity of 90% for many hours. The present invention also relates to a process for producing such magnet.
Permanent magnets of Fe-B-R types have been proposed as novel high-performance permanent magnets, which have magnetic properties beyond the maximum properties of the conventional rare earth-cobalt magnets and contain as the main components Fe, abundant light rare earth elements such as Nd and/or Pr and boron (B), without containing expensive elements Sm or Co (Japanese Patent Kokai-Publication Nos. 59-46008 and 59-89401 or corresponding EPA 101552).
The Curie temperature of the abovementioned magnetic alloy lies in general within a range of 300.degree. to 370.degree. C. However, when part of Fe is substituted with Co, it is possible to obtain an Fe-B-R type permanent magnet with a higher Curie temperature (Japanese Patent Kokai-Publication Nos. 59-64733 and 59-132104 or corresponding EPA 106948). Further, when part of R of the Fe-B-R type rare-earth permanent magnet containing Co and light rare-earth elements Nd and/or Pr as R is substituted with at least one of heavy rare-earth elements such as Dy, Tb, Ho, etc., it is possible to obtain a Co-containing Fe-B-R type rare-earth permanent magnet having a Curie temperature equal to or higher than the aforementioned Co-containing Fe-B-R type rare-earth permanent magnet, a high (BH) max beyond 25 MGOe and an improved temperature dependency, in particular, an improved iHc (Japanese Patent Kokai-Publication No. 60-34005, EPA 134304).
Although permanent magnets of the Fe-B-R type magnetic anisotropic sintered body have excellent magnetic properties, however, since these magnets contain as the main components rare-earth elements and iron readily oxidized in air into stable oxides, when used as magnetic circuit, results in the deterioration and fluctuation in magnetic characteristics of the magnetic circuits, and contaminates other peripheral devices due to oxides peeled off from the surface of the magnet.
To improve the corrosion resistance of the abovementioned Fe-B-R type permanent magnet, it has been proposed that the surface of the permanent magnet with an corrosion-resistant metallic film layer formed by electroless plating or electrolytic plating (Japanese Patent Kokai-Publication No. 58-162350). In this plating method, since the permanent magnet is of a sintered body having certain amount of pores, there exists another problem in that an acid or alkaline solution for pre-plating treatment resides within these pores and therefore the magnet material (sintered) body is corroded with the lapse of time. Further, since the magnet material body is poor in chemical resistance, there exists other problem in that the surface of the magnet material body is corroded in plating treatment and therefore the surface adhesive strength and the corrosion resistance of the plating layer are both not sufficient.
To overcome the abovementioned problems, it has been proposed a method of forming a metallic thin film on the surface of the sintered magnet material body by vapor plating to improve the corrosion resistance of the above Fe-B-R type permanent magnet (Japanese Patent Kokai-Publication Nos. 61-150201, 61-166115, 61-166116 and 61-166117 or corresponding U.S. Ser. No. 818,238 or EPA 0190461).
In these magnets, although the corrosion resistance of the Fe-B-R type permanent magnet can be improved, since the coated metal particles are deposited only on the surface of the magnet material body the adhesive strength is not sufficiently high. In particular, at the corners of a magnet body, the adhesive strength of the metallic particles is not uniform and therefore not high, thus resulting in various problems such as local thin film peeling off, local crack formation, local rust formation, when exposed to a severe environment for a long period of time.
On the other hand, with respect to the abovementioned Fe-B-R type permanent magnet whose surface is plated, since the permanent magnet body is a sintered body with pores the adhesive strength and the corrosion resistance are both poor. Further, the initial magnetic properties deteriorates by more than 10% after the magnet has been exposed at 60.degree. C. in an atmosphere of a relative humidity (R.H.) of 90% for 100 hours for corrosion test, thus indicating that the stability is not sufficient. Therefore there is much to be desired in the art.